1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constructed by thin-film transistors (hereinafter referred to as “TFT”), and a method of manufacturing the semiconductor device. For example, the present invention relates to an electrooptical device represented by a liquid crystal display panel and electronic equipment in which such an electrooptical device is mounted as a part.
In this specification, the semiconductor device means a general device that functions by using semiconductor characteristics, and an electrooptical device, a semiconductor circuit and electronic equipment are defined as a semiconductor device.
2. Description of the Related Art
Recently, much attention has been paid to a technique of forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of about several to several hundreds nm) formed on a substrate having an insulating surface. The thin film transistor has been widely applied to electrical devices such as IC, an electrooptical device, etc., and particularly developments to apply the thin film transistor to a switching element of an image display device have been rapidly required.
A liquid crystal display device is well known as an image display device. The active matrix type liquid crystal display device has been more frequently used than the passive type liquid crystal display device because higher-definition images can be provided by the active matrix type liquid crystal display device. In the active matrix type liquid crystal display device, a display pattern is formed on a screen by driving pixel electrodes arranged in a matrix form. More specifically, a voltage is applied across a selected pixel electrode and a counter electrode confronting the selected pixel electrode to optically modulate a liquid crystal layer disposed between the pixel electrode and the counter electrode, so that the optical modulation is recognized as a display pattern by a viewer.
Such active matrix type liquid crystal devices have been widely used in more diverse fields, and not only the large-area design of a screen size, but also high definition, high numerical aperture and high reliability design has been increasingly required. At the same time the enhancement of the productivity and the reduction in manufacturing cost have been also increasingly required.
When TFT is formed by using aluminum as a gate wiring material for TFT, malfunction of TFT and degradation in TFT characteristics are caused by formation of projections such as hillocks, whiskers, etc. due to a heat treatment and diffusion of aluminum atoms in a channel-forming region. On the other hand, when metal material having high resistance to the heat treatment, typically a metal element having a high melting point is used in order to avoid the above problem, there occurs another problem that the resistance of wires would increase if the screen size is increased, resulting in increase of power consumption, etc.